This invention relates to an analog-to-digital converter and, more particularly, to a parallel-type analog-to-digital converter which exhibits a high converting speed yet is formed of less component parts and thus is far more simplified than parallel-type analog-to-digital converters heretofore known.
Analog-to-digital converters, hereinafter referred to as A/D converters, generally are of two distinct types. A series-type A/D converter is of relatively simple construction in that it includes a small number of components. The analog signal is digitally encoded to produce a series of plural bits. However, this series-type A/D converter suffers from a relatively slow operating speed which is necessary in order to obtain all of the serial bits.
In the parallel-type A/D converter, the problem of slow operating speed is avoided. In such an A/D converter, the analog signal is converted to parallel plural bits in one operation. Thus, for an n-bit signal, the parallel-type converter operates at 1/n the speed of the series-type converter. However, in order to attain this high operating speed, the parallel-type A/D converter is formed of a large number of components, and thus of relatively complex construction.
In one form of parallel-type A/D converter, the input analog signal is sampled, and this sample is compared to each of a plurality of quantizing voltage levels. A digital code is produced corresponding to the highest quantizing level which is exceeded by the sampled voltage. To accomplish this, a number of voltage comparators must be provided, each comparator being supplied with a separate, discrete quantizing voltage level as well as with the sampled analog signal. It may be appreciated that, if an analog signal is to be converted into an n-bit digital code, then 2.sup.n -1 quantizing levels must be provided. This means that 2.sup.n -1 separate voltage comparators are needed. For a 4-bit code, fifteen voltage comparators must be used to quantize the analog signal. Because of the large number of components, and particularly voltage comparators, which are needed for a parallel-type A/D converter, such converters generally are used only if the digitally encoded signal contains a relatively small number of bits or if high conversion speed is of paramount importance. Accordingly, parallel-type A/D converters have been used for PCM communication applications and in transient signal recorders, as typical examples.
There is, therefore, a need for an A/D converter having a conversion speed on the order of the aforementioned parallel-type converters, yet is far less simplified, and thus less expensive.